這一章講的比特幣的錢包技術。通過這一章你可以理解，比特幣用什么樣的方式組織錢包里的密鑰，如何構建出安全密鑰空間。文章的技術細節比較多，閱讀需要一些耐心。
本章原文地址：
https://github.com/bitcoinbook/bitcoinbook/blob/develop/ch05.asciidoc
相關文章：
Wallets
The word "wallet" is used to describe a few different things in bitcoin.
At a high level, a wallet is an application that serves as the primary user interface. The wallet controls access to a user’s money, managing keys and addresses, tracking the balance, and creating and signing transactions.
抽象來看，錢包就是用戶界面，用于存儲管理用戶的比特幣，管理密鑰、地址，維護借貸平衡，完成交易簽名。具體而言，對程序員，錢包就是保存管理用戶密鑰的數據結構。
More narrowly, from a programmer’s perspective, the word "wallet" refers to the data structure used to store and manage a user’s keys.
In this chapter we will look at the second meaning, where wallets are containers for private keys, usually implemented as structured files or simple databases.
Wallet Technology Overview
In this section we summarize the various technologies used to construct user-friendly, secure, and flexible bitcoin wallets.
A common misconception about bitcoin is that bitcoin wallets contain bitcoin.?In fact, the wallet contains only keys.?The "coins" are recorded in the blockchain on the bitcoin network. Users control the coins on the network by signing transactions with the keys in their wallets.In a sense, a bitcoin wallet is a?keychain.
錢包里并沒有錢，錢包保存的是密鑰，錢存在區塊鏈上。比特幣錢包其實是密鑰串。
Tip?Bitcoin wallets contain keys, not coins. Each user has a wallet containing keys. Wallets are really keychains containing pairs of private/public keys (see [private_public_keys]). Users sign transactions with the keys, thereby proving they own the transaction outputs (their coins). The coins are stored on the blockchain in the form of transaction outputs (often noted as vout or txout).
There are two primary types of wallets,distinguished by whether the keys they contain are related to each other or not.
有兩種不同的比特幣錢包，取決于保存的密鑰是不是彼此關聯。第一種是叫JBOK（像臟話），隨機生成了一組密鑰，也叫非確定性錢包。
The first type is a?nondeterministic wallet, where each key is independently generated from a random number. The keys are not related to each other. This type of wallet is also known as a JBOK wallet from the phrase "Just a Bunch Of Keys."
The second type of wallet is a?deterministic wallet, where all the keys arederived from a single master key, known as the?seed. All the keys in this type of wallet are related to each other and can be generated again if one has the original seed. There are a number of different?key derivation?methods used in deterministic wallets. The most commonly used derivation method uses atree-like structureand is known as a?hierarchical deterministic?or?HD?wallet.
第二種錢包也稱作確定性錢包，里面所有的密鑰是從一個主密鑰導出的，形成了樹狀結構，也被稱作HD錢包。HD錢包的密鑰由一堆英語單詞編碼而來，這些也被稱作助記詞。
Deterministic wallets are initialized from a seed.To make these easier to use, seeds are encoded as English words, also known asmnemonic code words.
The next few sections introduce each of these technologies at a high level.
Nondeterministic (Random) Wallets
非確定性錢包也稱作隨機錢包，就是比如隨機生成100個私鑰，然后每次用一個。而這就意味著需要備份好每一個錢包，因為互不相關。而這樣也就意味著不能避免地址重用了。
In the first bitcoin wallet (now called Bitcoin Core), wallets were collections of randomly generated private keys. For example, the original Bitcoin Core clientpregenerates 100 random private keys when first started and generates more keys as needed, using each key only once.?Such wallets are being replaced with deterministic wallets because they are cumbersome to manage, back up, and import. The disadvantage of random keys is that if you generate many of them you must keep copies of all of them,?meaning that the wallet must be backed up frequently.?Each key must be backed up, or the funds it controls are irrevocably lost if the wallet becomes inaccessible.This conflicts directly with the principle of avoiding address reuse, by using each bitcoin address for only one transaction. Address reuse reduces privacy by associating multiple transactions and addresses with each other. A Type-0 nondeterministic wallet is a poor choice of wallet, especially if you want to avoid address reuse because it means managing many keys, which creates the need for frequent backups. Although the Bitcoin Core client includes a Type-0 wallet, using this wallet is discouraged by developers of Bitcoin Core.Type-0 nondeterministic (random) wallet: a collection of randomly generated keys?shows a nondeterministic wallet, containing a loose collection of random keys.
一般不建議用隨機錢包，除非作為測試使用。
TipThe use of nondeterministic wallets is discouraged for anything other than simple tests.?They are simply too cumbersome to back up and use. Instead, use an industry-standard–based HD wallet with a mnemonic seed for backup.
Figure 1. Type-0 nondeterministic (random) wallet: a collection of randomly generated keys
Deterministic (Seeded) Wallets
Deterministic, or "seeded," wallets are wallets that contain private keys that are all derived from a common seed, through the use of a one-way hash function. The seed is a randomly generated number that is combined with other data, such as an index number or "chain code" (seeHD Wallets (BIP-32/BIP-44)) to derive the private keys. In a deterministic wallet, the seed is sufficient to recover all the derived keys, and therefore a single backup at creation time is sufficient.The seed is also sufficient for a wallet export or import, allowing for easy migration of all the user’s keys between different wallet implementations.?Type-1 deterministic (seeded) wallet: a deterministic sequence of keys derived from a seedshows a logical diagram of a deterministic wallet.
確定性錢包會有一個種子，通過哈希算法依次生成密鑰，可以導出并且用于和其他錢包合并。
Figure 2. Type-1 deterministic (seeded) wallet: a deterministic sequence of keys derived from a seed
HD Wallets (BIP-32/BIP-44)
Deterministic wallets were developed to make it easy to derive many keys from a single "seed."The most advanced form of deterministic wallets is the HD wallet defined by the BIP-32 standard.?HD wallets contain keys derived in a tree structure, such that a parent key can derive a sequence of children keys, each of which can derive a sequence of grandchildren keys, and so on, to an infinite depth. This tree structure is illustrated inType-2 HD wallet: a tree of keys generated from a single seed.
最先進的確定錢包技術來自于BIP-32標準，采用一個種子生成一串密鑰的方法來管理。
Figure 3. Type-2 HD wallet: a tree of keys generated from a single seed
HD wallets offer two major advantages over random (nondeterministic) keys.First, the tree structure can be used to express additional organizational meaning, such as when a specific branch of subkeys is used to receive incoming payments and a different branch is used to receive change from outgoing payments.?Branches of keys can also be used in corporate settings, allocating different branches to departments, subsidiaries, specific functions, or accounting categories.
采用這種樹狀結構的密鑰生成，可以用于表示組織信息。例如，對于某個子節點下的密鑰都用來接收付款，其他的用于付款；或者公司也可以用不同的節點給不同的部門使用。
The second advantage of HD wallets is that users can?create a sequence of public keys without having access to the corresponding private keys.?This allows HD wallets to be used on an insecure server or in a receive-only capacity, issuing a different public key for each transaction. The public keys do not need to be preloaded or derived in advance, yet the server doesn’t have the private keys that can spend the funds.
第二個好處是，可以在不知道私鑰的情況下，生成一串公鑰，這樣即使在不安全的環境下，也是可以使用的。
Seeds and Mnemonic Codes (BIP-39)
HD wallets are a very powerful mechanism for managing many keys and addresses. They are even more useful if they are combined with a standardized way of creating seeds from a sequence of English words that are easy to transcribe, export, and import across wallets. This is known as a?mnemonic?and the standard is defined by BIP-39.Today, most bitcoin wallets (as well as wallets for other cryptocurrencies) use this standard and can import and export seeds for backup and recovery using interoperable mnemonics.
通過助記詞的方式生成密鑰，是可以在不同的錢包中備份和恢復的。
Let’s look at this from a practical perspective. Which of the following seeds is easier to transcribe, record on paper, read without error, export, and import into another wallet?
A seed for an deterministic wallet, in hex
0C1E24E5917779D297E14D45F14E1A1A? ? ? ?
A seed for an deterministic wallet, from a 12-word?mnemonic
army?van defense carry jealous true? ? ? ? ? ? ?
garbage claim echo media make crunch? ? ??
Wallet Best Practices
As bitcoin wallet technology has matured, certain common industry standards have emerged that make bitcoin wallets broadly interoperable, easy to use, secure, and flexible. These common standards are:
Mnemonic code words, based on BIP-39
HD wallets, based on BIP-32
Multipurpose HD wallet structure, based on BIP-43
Multicurrency and multiaccount wallets, based on BIP-44
比特幣錢包的最佳實踐，已經有一系列標準，參見上文列舉。
These standards may change or may become obsolete by future developments, but for now they form a set of interlocking technologies that have become the de facto wallet standard for bitcoin.
The standards have been adopted by a broad range of software and hardware bitcoin wallets, making all these wallets interoperable. A user can export a mnemonic generated on one of these wallets and import it in another wallet, recovering all transactions, keys, and addresses.
Some example of software wallets supporting these standards include (listed alphabetically) Breadwallet, Copay, Multibit HD, and Mycelium. Examples of hardware wallets supporting these standards include (listed alphabetically)?Keepkey, Ledger, and Trezor.
The following sections examine each of these technologies in detail.
TipIf you are implementing a bitcoin wallet, it should be built as?a HD?wallet, with a seed encoded as mnemonic code for backup, following the BIP-32, BIP-39, BIP-43, and BIP-44 standards, as described in the following sections.
Using a Bitcoin?Wallet
In[user-stories]we introduced Gabriel, an enterprising young teenager in Rio de Janeiro, who is running a simple web store that sells bitcoin-branded t-shirts, coffee mugs, and stickers.
Gabriel uses a Trezor bitcoin hardware wallet (A Trezor device: a bitcoin HD wallet in hardware) to securely manage his bitcoin.The Trezor is a simple USB device with two buttons that stores keys (in the form of an HD wallet) and signs transactions.Trezor wallets implement all the industry standards discussed in this chapter, so Gabriel is not reliant on any proprietary technology or single vendor solution.
這里介紹的是一種硬件比特幣錢包，可以用來保存密鑰并且簽名交易。
Figure 4. A Trezor device: a bitcoin HD wallet in hardware
When Gabriel used the Trezor for the first time, the device generated a mnemonic and seed from a built-in hardware random number generator. During this initialization phase, the wallet displayed a numbered sequence of words, one by one, on the screen (seeTrezor displaying one of the mnemonic words).
Figure 5. Trezor displaying one of the mnemonic words
在生成密碼的時候，硬件會按照順序顯示一串單詞，需要一個一個記下來。這些單詞可以用來恢復密碼。
By writing down this mnemonic, Gabriel created a backup (see?Gabriel’s paper backup of the mnemonic) that can be used for recovery in the case of loss or damage to the Trezor device. This mnemonic can be used for recovery in a new Trezor or in any one of the many compatible software or hardware wallets. Note that the sequence of words is important, so mnemonic paper backups have numbered spaces for each word. Gabriel had to carefully record each word in the numbered space to preserve the correct sequence.
Table 1. Gabriel’s paper backup of the mnemonic
Note?A 12-word mnemonic is shown in Gabriel’s paper backup of the mnemonic, for simplicity. In fact, most hardware wallets generate a more secure 24-word mnemonic. The mnemonic is used in exactly the same way, regardless of length. For the first implementation of his web store, Gabriel uses a single bitcoin address, generated on his Trezor device. This single address is used by all customers for all orders. As we will see, this approach has some drawbacks and can be improved upon with an HD wallet.
Wallet Technology Details
Let’s now examine each of the important industry standards that are used by many bitcoin wallets in detail.
這節課講比特幣錢包的技術細節。
Mnemonic Code Words (BIP-39)
Mnemonic code words are word sequences that represent (encode) a random number used as a seed to derive a deterministic wallet. The sequence of words is sufficient to re-create the seed and from there re-create the wallet and all the derived keys. A wallet application that implements deterministic wallets with mnemonic words will show the user a sequence of 12 to 24 words when first creating a wallet. That sequence of words is the wallet backup and can be used to recover and re-create all the keys in the same or any compatible wallet application.Mnemonic words make it easier for users to back up wallets because they are easy to read and correctly transcribe, as compared to a random sequence of numbers.
用了助記詞以后，就比隨機數好記許多了。
Tip?Mnemonic words are often confused with "brainwallets." They are not the same.The primary difference is that a brainwallet consists of words chosen by the user, whereas mnemonic words are created randomly by the wallet and presented to the user.?This important difference makes mnemonic words much more secure, because humans are very poor sources of randomness.
助記詞的安全性要比大腦錢包的安全性好，后者就是人在腦子里想一些詞語出來。但是人腦是很差勁的隨機源，也就是我們認知的范圍有限，想出的數往往不夠隨機。
Mnemonic codes are defined in BIP-39 (see[appdxbitcoinimpproposals]). Note that BIP-39 is one implementation of a mnemonic code standard. There is a different standard, with a different set of words, used by the Electrum wallet and predating BIP-39. BIP-39 was proposed by the company behind the Trezor hardware wallet and is incompatible with Electrum’s implementation. However,?BIP-39 has now achieved broad industry support across dozens of interoperable implementations and should be considered the de facto industry standard.
BIP-39被廣泛采納，已經被認為是工作的實際標準了。
BIP-39 defines the creation of a mnemonic code and seed, which we describe here in nine steps. For clarity, the process is split into two parts: steps 1 through 6 are shown inGenerating mnemonic wordsand steps 7 through 9 are shown inFrom mnemonic to seed.
生成密鑰分為兩步，第一步是生成助記詞，第二步是生成種子。
Generating mnemonic words
Mnemonic words are generated automatically by the wallet using the standardized process defined in BIP-39.The wallet starts from a source of entropy, adds a checksum, and then maps the entropy to a word list:
錢包先從一個隨機數開始，附上校驗，然后映射到一個詞表。
Create a random sequence (entropy) of 128 to 256 bits.
Create a checksum of the random sequence by taking the first (entropy-length/32) bits of its SHA256 hash.
Add the checksum to the end of the random sequence.
Split the result into 11-bit length segments.
Map each 11-bit value to a word from the predefined dictionary of 2048 words.
The mnemonic code is the sequence of words.
Generating entropy and encoding as mnemonic words?shows how entropy is used to generate mnemonic words.
Figure 6. Generating entropy and encoding as mnemonic words
Mnemonic codes: entropy and word lengthshows the relationship between the size of the entropy data and the length of mnemonic codes in words.
Table 2. Mnemonic codes: entropy and word length
From mnemonic to seed
The mnemonic words represent entropy with a length of 128 to 256 bits. Theentropy is then used to derive a longer (512-bit) seed through the use of the key-stretching function PBKDF2.?The seed produced is then used to build a deterministic wallet and derive its keys.
助記詞只代表了128到256比特的信息熵，而要生成512位的，需要采用PKBDF2函數把密鑰加長。加長主要通過兩個參數，一個是助記詞，一個是鹽。鹽的目的是增加破解的難度，防止構建彩虹表通過空間換時間破解。鹽還有另外一個目的就是可以通過口令短語的方式，提供多一層保護。
The key-stretching function takes two parameters:the mnemonic and asalt. The purpose of a salt in a key-stretching function is to make it difficult to build a lookup table enabling a brute-force attack.In the BIP-39 standard, the salt has another purpose—it allows the introduction of a passphrase that serves as an additional security factor protecting the seed, as we will describe in more detail in?Optional passphrase in BIP-39.
The process described in steps 7 through 9 continues from the process described previously inGenerating mnemonic words:
7.The first parameter to the PBKDF2 key-stretching function is the?mnemonic?produced from step 6.
8.The second parameter to the PBKDF2 key-stretching function is a?salt. The salt is composed of the string constant "mnemonic" concatenated with an optional user-supplied passphrase string.
9.PBKDF2 stretches the mnemonic and salt parameters using 2048 rounds of hashing with the HMAC-SHA512 algorithm, producing a 512-bit value as its final output. That 512-bit value is the seed. 這里采用的是SHA512算法來生成種子，進行2048輪哈希。
From mnemonic to seedshows how a mnemonic is used to generate a seed.
Figure 7. From mnemonic to seed
Tip?The key-stretching function, with its 2048 rounds of hashing, is a very effective protection against brute-force attacks against the mnemonic or the passphrase. It makes it extremely costly (in computation) to try more than a few thousand passphrase and mnemonic combinations, while the number of possible derived seeds is vast (2512).
Tables#mnemonic_128_no_pass,?#mnemonic_128_w_pass, and#mnemonic_256_no_passshow some examples of mnemonic codes and the seeds they produce (without any passphrase).
Table 3. 128-bit entropy mnemonic code, no passphrase,?resulting?seed
Table 4. 128-bit entropy mnemonic code, with?passphrase,?resulting?seed
Table 5. 256-bit entropy mnemonic code, no passphrase, resulting seed
Optional passphrase in BIP-39
The BIP-39 standardallows the use of an optional passphrase in the derivation of the seed.?If no passphrase is used, the mnemonic is stretched with a salt consisting of the constant string "mnemonic", producing a specific 512-bit seed from any given mnemonic. If a passphrase is used, the stretching function produces a?different?seed from that same mnemonic. In fact, given a single mnemonic, every possible passphrase leads to a different seed. Essentially, there is no "wrong" passphrase.All passphrases are valid and they all lead to different seeds, forming a vast set of possible uninitialized wallets.The set of possible wallets is so large (2512) that there is no practical possibility of brute-forcing or accidentally guessing one that is in use.
BIP-39還可以選擇采用短語口令生成種子。每個不同的短語口令，可以生成不同的種子，對應到巨大的錢包空間。
Tip?There are no "wrong" passphrases in BIP-39. Every passphrase leads to some wallet, which unless previously used will be empty.
The optional passphrase creates two important features:
A second factor (something memorized) that makes a mnemonic useless on its own, protecting mnemonic backups from compromise by a thief.
A form of plausible deniability or "duress wallet," where a chosen passphrase leads to a wallet with a small?amount?of funds used to distract an attacker from the "real" wallet that contains the majority of funds.
However, it is important to note that the use of a passphrase also introduces the risk of loss:
If the wallet owner is incapacitated or dead and no one else knows the passphrase, the seed is useless and all the funds stored in the wallet are lost forever.
Conversely, if the owner backs up the passphrase in the same place as the seed, it defeats the purpose of a second factor.
采用短語口令的好處就是，可快防止助記詞被盜用，但是壞處是如果你忘記了，就永遠丟失了。
While passphrases are very useful, they should only be used in combination with a carefully planned process for backup and recovery, considering the possibility of surviving the owner and allowing his or her family to recover the cryptocurrency estate.
Working with mnemonic codes
BIP-39 is implemented as a library in many different programming languages:
python-mnemonic
The reference implementation of the standard by the SatoshiLabs team that proposed BIP-39, in Python
bitcoinjs/bip39
An implementation of BIP-39, as part of the popular?bitcoinJS?framework, in JavaScript
libbitcoin/mnemonic
An?implementation of BIP-39, as part of the popular Libbitcoin framework, in C++
There is also a BIP-39 generator implemented in a standalone webpage, which is extremely useful for testing and experimentation.A BIP-39 generator as a standalone web pageshows a standalone web page that generates mnemonics, seeds, and extended private keys.
Figure 8. A BIP-39 generator as a standalone web page
The page (https://iancoleman.github.io/bip39/) can be used offline in a?browser,?or accessed online.
這一節講了BIP-39的實現，并介紹了一個頁面，可以用于直接生成密鑰。
Creating an HD Wallet from the Seed
HD wallets are created from a single?root seed, which is a 128-, 256-, or 512-bit random number. Most commonly, this seed is generated from a?mnemonic?as detailed in the previous section.
Every key in the HD wallet is deterministically derived from this root seed, which makes it possible to re-create the entire HD wallet from that seed in any compatible HD wallet. This makes it easy to back up, restore, export, and import HD wallets containing thousands or even millions of keys by simply transferring only the mnemonic that the root seed is derived from.
The process of creating the master keys and master chain code for an HD wallet is shown in?Creating master keys and chain code from a root seed.
Figure 9. Creating master keys and chain code from a root seed
The root seed is input into the HMAC-SHA512 algorithm and the resulting hash is used to create a?master private key?(m) and a?master chain code?(c).
在完成SHA512算法運算以后，把哈希的結果分成兩個部分，一個部分用于生成主私鑰m，一個部分用于生成主鏈碼c。m和G相乘就能得到主公鑰了。主鏈碼的目的為是了生成子密鑰。
The master private key (m) then generates a corresponding master public key (M) using thenormal elliptic curve multiplication process m * Gthat we saw in?[pubkey].
The chain code (c) is used to introduce entropy in the function that creates child keys from parent keys, as we will see in the next section.
Private child key derivation
HD wallets use a?child key derivation?(CKD) function to derive child keys from parent keys.
The child key derivation functions are based on a one-way hash function that combines:
A?parent private or public key (ECDSA uncompressed key)
A seed called a chain code (256 bits)
An index number (32 bits)
生成子密鑰有以下幾部分通過單向哈希函數構成：一個父私鑰，一個主鏈碼，一個索引數。需要確保的是，僅知道索引和某個子密鑰，無法導出其他子密鑰。
The chain code is used to introduce deterministic random data to the process, so that?knowing the index and a child key is not sufficient to derive other child keys.?Knowing a child key does not make it possible to find its siblings, unless you also have the chain code. The initial chain code seed (at the root of the tree) is made from the seed, while subsequent child chain codes are derived from each parent chain code.
These three items (parent key, chain code, and index) are combined and hashed to generate children keys, as follows.
The parent public key, chain code, and the index number are combined and hashed with the HMAC-SHA512 algorithm to produce a 512-bit hash.?This 512-bit hash is split into two 256-bit halves. The right-half 256 bits of the hash output become the chain code for the child. The left-half 256 bits of the hash are added to the parent private key to produce the child private key. InExtending a parent private key to create a child private key, we see this illustrated with the index set to 0 to produce the "zero" (first by index) child of the parent.
通過SHA512哈希生成的字符串，又進一步拆分成左右兩個部分，左部分與父私鑰相加，成為子私鑰；右部分直接作為主鏈碼，用于生成下一級的子私鑰。
Figure 10. Extending a parent private key to create a child private key
Changing the index allows us to extend the parent and create the other children in the sequence,e.g., Child 0, Child 1, Child 2, etc. Each parent key can have 2,147,483,647 (231) children (231?is half of the entire 232?range available because the other half is reserved for a special type of derivation we will talk about later in this chapter).
修改索引的數值就可以擴展父密鑰，依次生成子密鑰，重復這個過程，就可以一直生成下去。
Repeating the process one level down the tree, each child can?in turn?become a parent and create its own children, in an infinite number of generations.
Using derived child keys
Child private keys are indistinguishable from nondeterministic (random) keys. Because the derivation function is a one-way function, the child key cannot be used to find the parent key. The child key also cannot be used to find any siblings. If you have the nth?child, you cannot find its siblings, such as the n–1 child or the n+1 child, or any other children that are part of the sequence.Only the parent key and chain code can derive all the children.?Without the child chain code, the child key cannot be used to derive any grandchildren either. You need both the child private key and the child chain code to start a new branch and derive grandchildren.
采用生成子密鑰的方式，各個子節點是無法相互推出的，但是可以通過父密鑰以及主鏈碼，導出所有子密鑰。這些子密鑰可以用于生成公鑰和比特幣地址，這樣就可以對任意向該地址支付的交易進行簽名。
So what can the child private key be used for on its own?It can be used to make a public key and a bitcoin address. Then, it can be used to sign transactions to spend anything paid to that address.
Tip?A child private key, the corresponding public key, and the bitcoin address are all indistinguishable from keys and addresses created randomly. The fact that they are part of a sequence is not visible outside of the HD wallet function that created them. Once created, they operate exactly as "normal" keys.
Extended?keys
As?we saw earlier, the key derivation function can be used to create children at any level of the tree, based on the three inputs:a key, a chain code, and the index of the desired child.?The two essential ingredients are thekey and chain code,and combined these are called an?extended key. The term "extended key" could also be thought of as "extensible key" because such a key can be used to derive children.
Extended keys are stored and represented simply as the concatenation of the 256-bit key and 256-bit chain code into a 512-bit sequence. There are two types of extended keys. An extended private key is the combination of a private key and chain code and can be used to derive child private keys (and from them, child public keys).An extended public key is a public key and chain code, which can be used to create child public keys (public only), as described in[public_key_derivation].
擴展公鑰是由公鑰與鏈碼構成，用于創建子公鑰（僅公鑰）。
Think of an extended key as the root of a branch in the tree structure of the HD wallet. With the root of the branch, you can derive the rest of the branch.The extended private key can create a complete branch, whereas the extended public key canonly?create a branch of public keys.
擴展私鑰可以創建完整分支，但是擴展公鑰只能創建公鑰分支。
TipAn extended key consists of a private or public key and chain code. An extended key can create children, generating its own branch in the tree structure. Sharing an extended key gives access to the entire branch.
Extended keys are encoded using Base58Check, to easily export and import between different BIP-32–compatible wallets. The Base58Check coding for extended keys uses a special version number that results in the prefix "xprv" and "xpub" when encoded in Base58 characters to make them easily recognizable.Because the extended key is 512 or 513 bits,it is also much longer than other Base58Check-encoded strings we have seen previously.
擴展密鑰通常以xprv或者xpub作為前綴，密鑰長度為512或者513比特。
Here’s an example of an extended?private?key, encoded in Base58Check:
xprv9tyUQV64JT5qs3RSTJkXCWKMyUgoQp7F3hA1xzG6ZGu6u6Q9VMNjGr67Lctvy5P8oyaYAL9CAWrUE9i6GoNMKUga5biW6Hx4tws2six3b9c
Here’s the corresponding extended?public?key, encoded in Base58Check:
xpub67xpozcx8pe95XVuZLHXZeG6XWXHpGq6Qv5cmNfi7cS5mtjJ2tgypeQbBs2UAR6KECeeMVKZBPLrtJunSDMstweyLXhRgPxdp14sk9tJPW9
Public child key?derivation
As?mentioned previously, a very useful characteristic of HD wallets is the ability to derive public child keys from public parent keys,?without?having the private keys. This gives us two ways to derive a child public key: either from the child private key, or directly from the parent public key.
HD錢包可以直接從公鑰導出子公鑰，而不需要知道私鑰。
An extended public key can be used, therefore, to derive all of the?public?keys (and only the public keys) in that branch of the HD wallet structure.
This shortcut can be used to create very secure public key–only deployments where a server or application has a copy of an extended public key and no private keys whatsoever.That kind of deployment can produce an infinite number of public keys and bitcoin addresses, but cannot spend any of the money sent to those addresses.?Meanwhile, on another, more secure server, the extended private key can derive all the corresponding private keys to sign transactions and spend the money.
在僅導出子公鑰的場景下，可以把擴展公鑰部署在只需要放置公鑰的服務器上，這樣對應的比特幣地址只能收錢，不能花這些收到的錢。而在更安全的服務器上，卻可以生成對應的私鑰，處理這些收到的錢。
One common application of this solution is toinstall an extended public key on a web server that serves an?ecommerce?application.?The web server can use the public key derivation function to?create a new bitcoin address for every transaction?(e.g., for a customer shopping cart). The web server will not have any private keys that would be vulnerable to theft. Without HD wallets, the only way to do this is to generate thousands of bitcoin addresses on a separate secure server and then preload them on the?ecommerce?server. That approach is cumbersome and requires constant maintenance to ensure that the?ecommerce?server doesn’t "run out" of keys.
采用HD確定性錢包，只要安裝一個擴展公鑰在服務器上，作為電子商務的應用即可。這樣每來一位用戶，生成一個新的比特幣地址，對應到該用戶的購物車。如果不采用確定性錢包，那需要生成許多對公鑰提前部署，而且由于數目有限，還有可能把密鑰用完。
另外一個應用場景就是冷錢包。把私鑰離線存儲后，可以生成一個公鑰公布在網絡上，隨意收錢，只要私鑰離線保存即可。
Another common application of this solution is for cold-storage or hardware wallets.?In that scenario, the extended private key can be stored on a paper wallet or hardware device (such as a Trezor hardware wallet), while the extended public key can be kept online.The user can create "receive" addresses at will, while the private keys are safely stored offline.To spend the funds, the user can use the extended private key on an offline signing bitcoin client or sign transactions on the hardware wallet device (e.g., Trezor).Extending a parent public key to create a child public keyillustrates the mechanism for extending a parent public key to derive child public keys.
Figure 11. Extending a parent public key to create a child public key
Using an Extended Public Key on a Web Store
Let’s see how HD wallets are used by continuing our story with Gabriel’s web store.
Gabriel first set up his web store as a hobby, based on a simple hosted Wordpress page. His store was quite basic with only a few pages and an order form with a single bitcoin address.
Gabriel used the first bitcoin address generated by his Trezor device as the main bitcoin address for his store. This way, all incoming payments would be paid to an address controlled by his Trezor hardware wallet.
Customers would submit an order using the form and send payment to Gabriel’s published bitcoin address, triggering an email with the order details for Gabriel to process. With just a few orders each week, this system worked well enough.
However, the little web store became quite successful and attracted many orders from the local community. Soon, Gabriel was overwhelmed.With all the orders paying the same address, it became difficult to correctly match orders and transactions, especially when multiple orders for the same amount came in close together.
這里的案例中，如果采用同樣的地址收錢，那在收到許多筆錢的時候，就很難區分到底哪一筆錢是從哪里來的。于是可以采用另外一種方法，生成一系列的子公鑰，給每一個客戶，分配一個不同的地址。
Gabriel’s HD wallet offers a much better solution through the ability to derive public child keys without knowing the private keys. Gabriel can load an extended public key (xpub) on his website,which can be used to derive a unique address for every customer order.Gabriel can spend the funds from his Trezor, but the?xpub?loaded on the website can only generate addresses and receive funds. This feature of HD wallets is a great security feature. Gabriel’s website does not contain any private keys and therefore does not need high levels of security.
To export the?xpub, Gabriel uses the web-based software in conjunction with the Trezor hardware wallet. The Trezor device must be plugged in for the public keys to be exported. Note that hardware wallets will never export private keys—those always remain on the device.Exporting an?xpub?from a Trezor hardware wallet?shows the web interface Gabriel uses to export the?xpub.
Figure 12. Exporting an?xpub?from a Trezor hardware wallet
Gabriel copies the?xpub?to his web store’s bitcoin shop software. He uses?Mycelium Gear, which is an open source web-store plugin for a variety of web hosting and content platforms. Mycelium Gear uses the?xpub?to generate a unique address for every purchase.
Hardened child key?derivation
The?ability to derive a branch of public keys from an?xpub?is very useful, but it comes with a potential risk. Access to an?xpub?does not give access to child private keys. However, because the?xpub?contains the chain code,if a child private key is known, or somehow leaked, it can be used with the chain code to derive all the other child private keys.A single leaked child private key, together with a parent chain code, reveals all the private keys of all the children. Worse, the child private key together with a parent chain code can be used to deduce the parent private key.
采用子密鑰導出的方式有一個風險，一旦子私鑰泄露以后，可以根據鏈碼推測出所有的子私鑰。于是采用了一個強化的替換方法，用父私鑰導出子鏈碼， 而不是導出父公鑰。
To counter this risk, HD wallets use an alternative derivation function called?hardened derivation, which "breaks" the relationship between parent public key and child chain code. The hardened derivation function uses theparent private key to derive the child chain code, instead of the parent public key.This creates a "firewall" in the parent/child sequence, with a chain code that cannot be used to compromise a parent or sibling private key. The hardened derivation function looks almost identical to the normal child private key derivation, except that the parent private key is used as input to the hash function, instead of the parent public key, as shown in the diagram inHardened derivation of a child key; omits the parent public key.
Figure 13. Hardened derivation of a child key; omits the parent public key
When the hardened private derivation function is used, the resulting child private key and chain code are completely different from what would result from the normal derivation function.The resulting "branch" of keys can be used to produce extended public keys that are not vulnerable, because the chain code they contain cannot be exploited to reveal any private keys. Hardened derivation is therefore used to create a "gap" in the tree above the level where extended public keys are used.
采用強化的密鑰生成方式，產生的子私鑰與鏈碼，與普通的生成方式有很大不同。
In simple terms, if you want to use the convenience of an?xpub?to derive branches of public keys, without exposing yourself to the risk of a leaked chain code, you should derive it from a hardened parent, rather than a normal parent.As a best practice, the level-1 children of the master keys are always derived through the hardened derivation, to prevent compromise of the master keys.
一般而言，主密鑰的第一層級子密鑰通常采用強化的導出方式，以防止泄密造成的損失。
Index numbers for normal and hardened?derivation
The?index number used in the derivation function is a 32-bit integer. To easily distinguish between keys derived through the normal derivation function versus keys derived through hardened derivation,this index number is split into two ranges. Index numbers between 0 and 231–1 (0x0 to 0x7FFFFFFF) are used?only?for normal derivation. Index numbers between 231?and 232–1 (0x80000000 to 0xFFFFFFFF) are used?only?for hardened derivation. Therefore, if the index number is less than 231, the child is normal, whereas if the index number is equal or above 231, the child is hardened.
為了區分兩種不同的索引，把32比特的數值拆分成兩半，以231為分界，如果小于這個值，就是正常的導出；如果大于或者等于，就是強化的導出形式。具體到表示形式上，采用正常的導出格式，以0打頭，采用強化的導出格式，采用0’的方式表示 。
To make the index number easier to read and display,the index number for hardened children is displayed starting from zero, but with a prime symbol.?The first normal child key is therefore displayed as 0, whereas the first hardened child (index 0x80000000) is displayed as 0'. In sequence then, the second hardened key would have index 0x80000001 and would be displayed as 1', and so on. When you see an HD wallet index i', that means 231+i.
HD wallet key identifier (path)
Keys in an HD wallet are identified using a "path" naming convention, with each level of the tree separated by a slash (/) character (seeHD wallet path examples). Private keys derived from the master private key start with "m." Public keys derived from the master public key start with "M." Therefore, the first child private key of the master private key is m/0. The first child public key is M/0. The second grandchild of the first child is m/0/1, and so on.
HD錢包中，采用斜杠的方式代表層級順序，即生成的密鑰之間的關系。例如， m/x/y/z格式的密鑰，就代表了m/x/y的第z個子密鑰，而m/x/y是m/x的第y個子密鑰，而m/x是m的第x個子密鑰。
The "ancestry" of a key is read from right to?left,?until you reach the master key from which it was derived. For example, identifier m/x/y/z describes the key that is the z-th?child of key m/x/y, which is the y-th?child of key m/x, which is the x-th?child of m.
通過以下的表格，你可以看到通過一種簡單的方式，就可以把密鑰間的層級關系很清晰的梳理了出來。
Table 6. HD wallet path examples
Navigating the HD wallet tree?structure
The?HD wallet tree structure offers tremendous flexibility.Each parent extended key can have 4 billion children: 2 billion normal children and 2 billion hardened children.Each of those children can have another 4 billion children, and so on. The tree can be as deep as you want, with an infinite number of generations. With all that flexibility, however, it becomes quite difficult to navigate this infinite tree. It is especially difficult to transfer HD wallets between implementations, because the possibilities for internal organization into branches and subbranches are endless.
HD錢包可以擴展出40億子密鑰，普通模式與強化模式各一半。由于空間巨大，要遍歷的話存在困難，于是BIP-43出現，提議把強化導出的節點作為一個定位符來指定樹的結構與作用。例如，采用m/i'/來代表第i個子密鑰下面的所有密鑰，具備特定的作用。
這有點像我們有一個無窮大的樹狀結構，從某個節點i開始，劃分功能，其下面所有的節點都具備某特定的功能。
Two BIPs offer a solution to this complexity by creating some proposed standards for the structure of HD wallet trees.BIP-43 proposes the use of the first hardened child index as a special identifier that signifies the "purpose" of the tree structure.Based on BIP-43, an HD wallet should use only one level-1 branch of the tree, with the index number identifying the structure and namespace of the rest of the tree by defining its purpose. For example, an HD wallet using only branch m/i'/ is intended to signify a specific purpose and that purpose is identified by index number "i."
Extending that specification, BIP-44 proposes a?multiaccount?structure as "purpose" number 44' under BIP-43. All HD wallets following the BIP-44 structure are identified by the fact thatthey only used one branch of the tree: m/44'/.
在BIP-44的規范下，錢包只使用根密鑰下的第45級子密鑰。采用以下規范，錢包就可以在不同的幣種間，劃分出不同的空間。例如比特幣選用了 m/44'/0', Bitcoin Testnet 選用m/44'/1', 萊特幣選用的是Litecoin is m/44'/2'。
BIP-44 specifies the structure as consisting of five predefined tree levels:
m / purpose' / coin_type' / account' / change / address_index
The first-level "purpose" is always set to 44'. The second-level "coin_type" specifies the type of cryptocurrency coin, allowing for multicurrency HD wallets where each currency has its own subtree under the second level. There are three currencies defined for now: Bitcoin is m/44'/0', Bitcoin Testnet is m/44'/1', and Litecoin is m/44'/2'.
The third level of the tree is "account,"which allows users to subdivide their wallets into separate logical subaccounts, for accounting or organizational purposes.?For example, an HD wallet might contain two bitcoin "accounts": m/44'/0'/0' and m/44'/0'/1'. Each account is the root of its own subtree.
再往下就是把每個用戶的空間分成不同的邏輯空間，用于不同的會計目的，或者針對不同的機構使用。再往下一層，可以繼續劃分，用于收錢的地址或者用于找零錢的地址。到第五層就是錢包導出的實際使用的地址了。
On the fourth level, "change," an HD wallethas two subtrees, one for creating receiving addresses and one for creating change addresses.?Note that whereas the previous levels used hardened derivation, this level uses normal derivation. This is to allow this level of the tree to export extended public keys for use in a nonsecured environment.Usable addresses are derived by the HD wallet as children of the fourth level, making the fifth level of the tree the "address_index."For example, the third receiving address for bitcoin payments in the primary account would be M/44'/0'/0'/0/2.BIP-44 HD wallet structure examplesshows a few more examples.
到此就建立起了比特幣錢包里公私鑰的層級關系以及對應生成的密鑰地址的路徑了。
Table 7. BIP-44 HD wallet structure examples
